Is it possible to send separate mono guitar/vocal signals out via USB from the Play GTX to my PC/Mac for recording in a DAW, for example?
Unfortunately no. The Play GTX's USB can only support a 2 channel stereo in/out, which will always be a mix of guitar and vocal. Therefore it is not possible to receive the vocal and guitar signals both simultaneously AND separately into your PC or Mac via USB. You can do one or the other, but not both.
Therefore, if you wish to process multiple stereo tracks, you'd need to send vocal/guitar in separate passes and record the stereo results for each.